A wide variety of medical endoprostheses or implants are known from the prior art for a wide range of applications. Within the context of the present invention, implants are to be understood to be endovascular prostheses or other endoprostheses, for example stents (vessel stents (vascular stents, including stents for application in the area of the heart and heart valve stents, for example mitral valve stents and pulmonary valve stents), bile duct stents), endoprostheses for closing patent foramen ovale (PFO), stent grafts for treating aneurysms, endoprostheses for closing an ASD (atrial septal defect), and prostheses in the area of hard and soft tissue.
Nowadays, stents used for treatment of stenoses (vessel constrictions) are used particularly frequently as implants. They have an open-worked hollow cylindrical (tubular) main structure, which is open at both longitudinal ends.
The main structure of conventional stents is often composed of individual meshes, which are formed of a multiplicity of crosspieces (struts), for example of crosspieces forming a zigzagged or meandering structure. An implant of this type is often introduced by means of a catheter into the vessel to be treated and is used to support the vessel over a relatively long period of time (months to years). Due to the use of stents, constricted areas in the vessels can be expanded, thus resulting in a lumen gain.
Stents or other implants normally adopt two states: namely a compressed state with a small diameter and an expanded state with a larger diameter. In the compressed state, the implant can be introduced by means of a catheter into the vessel to be supported and can be positioned at the point to be treated. To this end, the implant is crimped onto the balloon of a catheter for example. The implant is then dilated at the treatment location, for example by means of the balloon of the catheter, and then adopts the expanded state, in which the implant remains in the vessel and supports the vessel, once the catheter with the balloon has been withdrawn again from the body of the patient being treated.
With the use of implants, in particular stents, in hard tissue (for example as vertebral body stents), crosspieces having a very large diameter are currently used so as to achieve the required rigidity, and therefore the tissue structure surrounding the stent is stabilized securely by the stent. Due to the large crosspiece dimensions, implants of this type have a very large diameter on the whole, which is obstructive however in the event of insertion through narrow access paths. The use of stents having a very high level of radial rigidity is also desirable in the case of very severely calcified vessel stenoses. By contrast, implants having a small diameter and a high level of flexibility are required in particular in the case of very severely stenosed vessels or complete closures so as to enable easy placement of the implant without pre-dilation. However, these contrasting properties cannot be implemented by a conventional stent design, or can only be implemented with severe limitations.
Various embodiments of stents have long been known, for example braided wire stents and slotted tube stents, with different cell designs (closed cell design, open cell design or mixtures of both types). Stents with thick crosspieces and closed cell design generally have a high level of radial rigidity, but poor deliverability. By contrast, stents with thin crosspieces and open cell design often have good deliverability, but are less radially rigid.
The term “deliverability” describes the capacity of a stent to be supplied and placed. Deliverability can be measured and described by a number of parameters, for example on the basis of the capacity of the stent to pass through curves or the ability of the stent to move through narrow points. Deliverability is also manifested in the maneuverability of the system.
In the present invention, radial rigidity is understood to be a resistance of an implant (or implant portion) against a force acting in the radial direction, from the outside in with respect to the implant.
Document EP 0 830 853 A1 discloses a stent that is provided, in particular, for application in the area of an ostium. For this application, a relatively high level of radial rigidity is often not necessary over the entire length of the implant, but in particular at the end portion of the stent. The aforementioned document discloses a stent in which for example, in the desired region of the stent, crosspieces are shorter in length compared to other crosspieces, the main structure consists of a material different from that of the remaining part of the main structure, a connecting crosspiece with a U-shape is replaced by a radially more rigid Z-shape or S-shape, or the crosspieces have less material, that is to say are thinner, at least along the other portions. Alternatively, the crosspieces in the desired region may also have more material, that is to say may be thicker. The above solutions are often insufficient in terms of radial rigidity and also have a negative effect on deliverability.